Methods for Adapting Density of Demodulation Reference Signals

ABSTRACT

According to an aspect, a wireless node selects a set of reference signal antenna ports for use in transmitting data to other wireless nodes in a given transmit time interval, from a plurality of sets of reference signal antenna ports that are available for use and that include reference signal antenna ports having different reference signal densities in the frequency and/or time dimension. The wireless node sends a message to a second wireless node indicating a reference signal assignment and including an indication of the selected set of reference signal antenna ports.

TECHNICAL FIELD

The present application is generally related to wireless communicationnetworks and is more particularly related to techniques for adapting thedensity of demodulation reference signals transmitted in such networks.

BACKGROUND

The 3^(rd)-Generation Partnership Project (3GPP) is developing standardsfor a so-called fifth-generation (5G) wireless communication system, tosupport ever-increasing demands for improved bandwidths andavailability. Initial proposals for the access network portion of thissystem, i.e., the radio access network (RAN) portion of the 5G system,have referred to this new RAN with names such as “New Radio” (NR), or“Next Generation” (NG).

Regardless of the name, the physical layer of 5G mobile radio systems isexpected to handle a vast number of different transmission scenariosthat follow from the systems' support of multiple transmissionnumerologies, variable data transmission time intervals, and earlydecoding for latency-critical applications. These new scenarios impose aneed for the physical layer to be even more flexible than was the casewhen the fourth-generation RAN referred to as Long Term Evolution (LTE)was first designed. In addition to these new transmission scenarios, thephysical layer of a 5G system should handle, as it does in LTE,different transmission characteristics in terms of large variations insignal-to-interference-plus-noise ratio (SINR), Doppler, delay spreads,and channel richness.

As of the development of 4^(th)-generation (4G) networks, theconventional way of designing reference signals for coherentdemodulation of physical layer control and data channels signals is todefine a few reference signal patterns that can be configured for acertain transmission mode or for a certain transmit antenna setup. InLTE, downlink reference signals known as cell-specific reference signals(CRS) are broadcast in each cell, for use by all wireless devices(referred to as user equipments, or UEs, in 3GPP terminology) inmeasuring and estimating the characteristics of the downlink propagationchannel. FIG. 1 illustrates the CRS patterns within an LTE slot for thecases of 1, 2 or 4 transmit antennas in downlink. It can be seen in thefigure that CRS symbols for each one of up to four antenna ports(labeled AP0, AP1, AP2, and AP3) are transmitted in mutually exclusivesets of resource elements in the LTE slot. In the multi-port scenarios(2 Tx antennas and 4-antenna scenarios), it can be seen that from eachtransmit (TX) antenna port, “zero power” symbols are transmitted in theresource elements that are used for CRS symbols by any other antennaport.

These CRS patterns are statically configured and cannot be adapted toradio link conditions, as they are broadcast, and not UE-specific.However, LTE also supports UE-specific demodulation reference signals(DM-RS) in the downlink that, to some extent, can adapt to radio linkconditions. These are especially useful for facilitating the measurementof radio link conditions to enable transmissions and reception ofmultiple MIMO (Multiple-Input Multiple-Output) layers.

FIG. 2 shows the DM-RS patterns used in LTE transmission modes 9 and 10,in which the DM-RS mapping to resource elements of the OrthogonalFrequency-Division Multiplexing (OFDM) time-frequency grid depends onthe number of transmission layers. LTE supports up to 8-layertransmission in downlink by using a Walsh-Hadamard Orthogonal Cover Code(OCC) in time of either length-2 (up to 4-layers) or of length-4 (formore than 4-layers). In the case of length-4 OCC, the cover code isapplied across two consecutive slots that define an LTE subframe andwhich also corresponds to a transmission time interval (TTI) of a datachannel in LTE. The use of OCC is in FIG. 2 illustrated by a dashed boxand in the case of length-2 OCC the two cover codes are [1 1] and [1−1].

In the case of up to two layers, illustrated at the left side of FIG. 2,the DM-RS for each of one or two antenna ports is transmitted using sixresource elements in each LTE slot. If two layers are transmitted, theDM-RS for each antenna port are transmitted in each slot using the samesix resource elements, with the DM-RS for the two antenna ports beingseparated from one another by the orthogonal cover codes. In the case ofmore than two layers, as illustrated to the right side of FIG. 2, twoslots are used together—this allows a length-two OCC in each slot to beeffectively extended, into a length-four OCC. In the case of eightlayers, then, four of the antenna ports transmit their respectiveDM-RS's on a first set of 12 resource elements in the two slots, withthe DM-RS's being separated by length-four OCC's extending across thetwo slots. Likewise, the remaining four ports transmit their respectiveDM-RS's on a second set of 12 resource elements in the two slots, withthe DM-RS's again being separated by length-four OCC's extending acrossthe two slots. The OCC for the DM-RS for AP0 and AP2 (which aretransmitted in different sets of resource elements) is [1 1 1 1]; theOCC for the DM-RS for AP1 and AP3 (which are transmitted in first andsecond sets of resource elements, respectively) is [1 −1 1 −1]; the OCCfor the DMR-RS for AP4 and AP5 (transmitted in the first and second setsof resource elements, respectively) is [1 1 −1 −1]; and the DM-RS forAP6 and AP7 (the first and second sets of resource elements,respectively) is [1 −1 −1 1].

The LTE demodulation reference signal patterns were not designed tohandle the latency-critical transmissions addressed in 5G. Inparticular, 5G systems are expected to require that DM-RS is transmittedat or near the beginning of each TTI, to enable early decoding. As canbe seen in FIG. 2, the LTE DM-RS design places the DM-RS symbols at theend of each slot.

FIG. 3 shows a DM-RS structure that has been proposed for 5G to meetrequirements of early decoding. In this structure, up to 8-layer MIMOtransmissions are supported, where transmissions of DM-RS for up to fourlayers is done via a so-called 4 Combs, i.e., interleaved FDM with aninterleaving distance of four subcarriers, whereas MIMO transmissions ofmore than 4-layers are done by introducing length-2 OCC in frequency perComb. With this DM-RS structure, the demodulation and decoding of datacould start almost directly after receiving the second OFDM symbol. Inthe illustration depicted in FIG. 3, a physical resource block (PRB) of12 subcarriers has been assumed, which implies that a length-2 OCC infrequency relies on PRB bundling, as the cover code introducesdependencies in frequency domain between two consecutive PRBs.

The left-hand of FIG. 3 illustrates the location of the DM-RS forantenna ports 0 and 4. The DM-RS for these two ports occupy the same sixresource elements in the PRB bundle (extending across 24 subcarriers),and are separated from one another by length-2 OCCs. Moving to theright, FIG. 3 next illustrates the DM-RS configuration for antenna ports1 and 5—as seen in the figure, these occupy different resource elementsfrom those used for antenna ports 0 and 4, and are in factfrequency-multiplexed with those resource elements. Again, the DM-RS forantenna ports 1 and 5 are separated from one another by length-two OCCs.Moving further to the right, FIG. 3 illustrates the DM-RS configurationfor antenna ports 2 and 6, and then for antenna ports 3 and 7. The PRBbundling and the frequency multiplexing of the DM-RS allow all of theDM-RS symbols to be transmitted early in the time slot—in theillustrated approach, the DM-RS resides in the second OFDM symbol. Thisallows early reception of the DM-RS and fast channel estimation, forlatency-critical applications.

SUMMARY

A problem with the solutions illustrated in FIGS. 2 and 3 and withsimilar solutions is that they produce an undesirable tradeoff—eitherthe DM-RS patterns cannot meet the 5G requirements of early decoding, orthe proposed DM-RS patterns for meeting the requirements for earlydecoding have too sparse a density for users that do not havesufficiently high SINR (e.g., cell edge users) or sufficiently goodradio conditions for higher-rank transmissions (i.e., transmissions witha higher number of spatial layers). To efficiently support such users, adenser pattern is needed. However, such a denser pattern would generallyrequire additional overhead, because of the additional resource elementsneeded for the pattern. Moreover, it is a problem to determine how totransmit information with different requirements on error probabilitysimultaneously, in the same physical resource, with a common DM-RSstructure.

On way to address different needs for DM-RS density, or other referencesignal density, is to make the DM-RS density adaptable, so that it canbe changed from one time-slot to another. However, the currentsemi-static configurations of DM-RS density are not sufficient to allowrapid adaptation to different requirements. Further, introducingadaptability of the DM-RS requires additional signaling. Still further,it is also a problem to determine how to adapt the DM-RS density to UEsin multi-user-MIMO (MU-MIMO) scheduling.

Various embodiments of the techniques and apparatus described hereinaddress one or several of these problems by providing a solution inwhich an adaptable reference signal density is indicated and transmittedto the UE in a structured way, by merging antenna reference signal orantenna ports of lower density. In this way, the density can be adapteddepending on the current needs. For example, in MU-MIMO scheduling, somescheduled UEs may need a higher reference signal density than others.The presently disclosed techniques facilitate efficient scheduling insuch scenarios.

As described in detail below, one way to signal the adaptive referencesignal density to the UE is by utilizing the fact that the signaling ofhigher reference signal density can simultaneously indicate a totaltransmitted rank restriction. In other words, by utilizing thecorrelation between the presence of low-SINR users and the best choiceof rank, a lower overhead signaling solution is possible in the downlinkcontrol information (DCI) format.

As described in further detail below, in some embodiments of thepresently disclosed techniques, a mobile device is semi-staticallyconfigured or dynamically indicated from the scheduling downlink controlmessage, or a combination of both, with a reference signal pattern tosupport the envelope of the requirements of a current population ofusers. The densification of a reference signal is obtained by mergingone or multiple reference signals (or ports) of lower density into newreference signals (or ports).

Accordingly, adapting for different densities of reference signalantenna ports may be effectuated by selecting of one or more sets ofreference signal antenna ports from a plurality of available sets ofreference signal antenna ports (possible configurations of referencesignal antenna ports), where at least two of the sets in the pluralityof sets of reference signal antenna ports have different densities.Therefore, the plurality of possible sets of reference signal antennaports that may be selected for transmission or reception includes atleast one or more sets of reference signal antenna ports arranged in aphysical resource block (PRB) to have a relatively lower density(unmerged) and one or more sets of reference signal antenna portsarranged in a PRB to have a higher density (as if merged from PRBs withlower densities). Some sets in the plurality of sets of reference signalantenna ports may include even higher densities of reference signalantenna ports, as described below.

Effectively, the total number of reference signal ports (i.e., themaximum possible rank) is reduced, when higher densities are selected.For the purposes of illustration, the detailed description belowconsiders the DM-RS structure shown in FIG. 3, where 4 combs are used inconjunction with length-2 OCC's, to be the baseline configuration forsupporting up to 8 layers/antenna ports. It will be appreciated that thedisclosed techniques can be applied to the adaptive densification ofother baseline multi-port DM-RS configurations or other reference signalconfigurations, however.

According to some embodiments, a method, in a first wireless node,includes selecting a set of reference signal (e.g., DM-RS) antenna portsfor use in transmitting data to or receiving data from one or more otherwireless nodes in a given transmit time interval, from a plurality ofsets of reference signal antenna ports that are available for use. Theplurality of sets of reference signal antenna ports include referencesignal antenna ports having different reference signal densities in thefrequency and/or time dimension. The method also includes sending amessage (e.g., scheduling message) to a second wireless node indicatinga reference signal assignment to the second wireless node. The messageincludes an indication of the selected set of reference signal antennaports.

According to some embodiments, a method, in a second wireless node,includes receiving, from a first wireless node, a message indicating areference signal assignment to the second wireless node. The messageincludes an indication of a set of reference signal antenna portsselected from a plurality of available sets of reference signal antennaports known to the second wireless node, and the plurality of sets ofreference signal antenna ports include reference signal antenna portshaving different reference signal densities in the frequency dimensionand/or time dimension. The method further includes identifying theindicated set of reference signal antenna ports from the receivedindication.

According to some embodiments, a first wireless node is adapted toselect a set of reference signal antenna ports for use in transmittingdata to or receiving data from one or more other wireless nodes in agiven transmit time interval, from a plurality of sets of referencesignal antenna ports that are available for use. The plurality of setsof reference signal antenna ports include reference signal antenna portshaving different reference signal densities in the frequency dimension,the time dimension, or both. The first wireless node is also adapted tosend a message to a second wireless node indicating a reference signalassignment to the second wireless node. The message includes anindication of the selected set of reference signal antenna ports.

According to some embodiments, a second wireless node is adapted toreceive, from a first wireless node, a message indicating a referencesignal assignment to the second wireless node, where the messageincludes an indication of a set of reference signal antenna portsselected from a plurality of available sets of reference signal antennaports known to the second wireless node, where the plurality of sets ofreference signal antenna ports include reference signal antenna portshaving different reference signal densities in the frequency dimensionand/or time dimension. The second wireless node is also adapted toidentify the indicated set of reference signal antenna ports from thereceived indication.

According to some embodiments, a first wireless node configured foroperation in a wireless communication network includes a transceivercircuit and a processing circuit operatively coupled to the transceivercircuit. The processing circuit is configured to select a set ofreference signal antenna ports for use in transmitting data to orreceiving data from one or more other wireless nodes in a given transmittime interval, from a plurality of sets of reference signal antennaports that are available for use, where the plurality of sets ofreference signal antenna ports include reference signal antenna portshaving different reference signal densities in the frequency dimensionand/or time dimension. The processing circuit is also configured to sendto a second wireless node, via the transceiver circuit, a messageindicating a reference signal assignment to the second wireless node,where the message includes an indication of the selected set ofreference signal antenna ports.

According to some embodiments, a second wireless node configured foroperation in a wireless communication network includes a transceivercircuit and a processing circuit operatively coupled to the transceivercircuit. The processing circuit is configured to receive from a firstwireless node, via the transceiver circuit, a message indicating areference signal assignment to the second wireless node, where themessage includes an indication of a set of reference signal antennaports selected from a plurality of available sets of reference signalantenna ports known to the second wireless node. The plurality of setsof reference signal antenna ports include reference signal antenna portshaving different reference signal densities in the frequency dimensionand/or time dimension. The processing circuit is also configured toidentify the indicated set of reference signal antenna ports from thereceived indication.

The processing circuit is configured to carry out any example of themethod or apparatus described, in some examples, in association with thetransceiver circuit.

Further aspects of the present invention are directed to an apparatus,computer program products or computer readable storage mediumcorresponding to the methods summarized above and functionalimplementations of the above-summarized apparatus and wireless device.

Details of various methods and apparatuses are provided below. Inaddition, an enumerated list of example embodiments of the presentlydisclosed techniques and apparatuses is provided.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates CRS patterns in LTE.

FIG. 2 shows UE-specific DM-RS patterns in LTE downlink.

FIG. 3 illustrates a DM-RS structure targeting early decoding.

FIG. 4 is a block diagram illustrating an example network node, such asa base station.

FIG. 5 is a process flow diagram illustrating a method in the networknode, according to some embodiments.

FIG. 6 is a block diagram illustrating an example wireless device, suchas a UE.

FIG. 7 is a process flow diagram illustrating a method in the wirelessdevice, according to some embodiments.

FIG. 8A illustrates a table that is an example of mapping using twoindicator bits to indicate a set of available ports, according to someembodiments.

FIG. 8B illustrates an example antenna port table with port numbersreferring to the DM-RS densities exemplified in FIGS. 9 to 11, accordingto some embodiments.

FIG. 9 shows baseline rank-8 DM-RS patterns densified to rank-6 DM-RSpatterns, according to some embodiments.

FIG. 10 shows rank-6 DM-RS patterns densified to rank-4 DM-RS patterns,according to some embodiments.

FIG. 11 shows rank-4 DM-RS patterns densified to a rank-2 DM-RS pattern,according to some embodiments.

FIG. 12 shows baseline rank-8 DM-RS pattern densified to an alternativerank 6 pattern, according to some embodiments.

FIG. 13 is a process flow diagram illustrating another method in thewireless device, according to some embodiments.

FIG. 14 is a block diagram illustrating a functional representation of amethod in the network node, according to some embodiments.

FIG. 15 is a block diagram illustrating a functional representation of amethod in the wireless device, according to some embodiments.

DETAILED DESCRIPTION

The presently disclosed techniques are described in the context ofimprovements to the LTE wireless communications standards, as might beadopted in standards for a 5G wireless communications system used inconjunction with or to replace the existing LTE systems. Moreparticularly, the presently disclosed techniques are described by usingan Orthogonal Frequency-Division Multiple Access (OFDMA) signalstructure like that used in LTE, e.g., with twelve subcarriers perphysical resource block (PRB) and seven OFDM symbols per slot (assuminga “normal” cyclic prefix), etc. It should be understood, however, thatthe techniques are more generally applicable to other wirelesscommunications networks and other OFDMA signal structures.

Furthermore, it will be appreciated that the terms “user equipment” or“UE” are used herein for convenience, as these are commonly used in 3GPPdocumentation. For the purpose of understanding the scope of thepresently disclosed techniques and apparatuses, however, these termsshould be understood more generally, as referring to wireless devicesconfigured to operate as access terminals in a wireless communicationnetwork, whether those wireless devices are consumer-oriented devicessuch as cellular telephones, smartphones, wireless-equipped laptops,tablets, or the like, or machine-to-machine (M2M) devices for use inindustrial applications or in enabling the Internet of Things (IoT).Likewise, the terms eNB and eNodeB, as used herein, should be understoodto refer generally to base stations or wireless access nodes in awireless communications system.

As discussed above, a problem with the solutions illustrated in FIGS. 2and 3 for transmitting DM-RS is that they produce an undesirabletradeoff—either the DM-RS patterns cannot meet the 5G requirements ofearly decoding, or the proposed DM-RS patterns for meeting therequirements for early decoding have too sparse a density for users thatdo not have sufficiently high SINR (e.g., cell edge users) orsufficiently good radio conditions for higher-rank transmissions (i.e.,transmissions with a higher number of spatial layers). To efficientlysupport such users, a denser pattern would be needed. However, such adenser pattern would generally require additional overhead, because ofthe additional resource elements needed for the pattern. Moreover, it isa problem to not know how to transmit information with differentrequirements on error probability simultaneously, in the same physicalresource, with a common DM-RS structure.

Handling different needs for DM-RS density can be addressed by makingthe DM-RS density adaptable, e.g., so that it can be changed from onetime-slot to another. Introducing adaptability of the DM-RS generallyrequires additional signaling. Further, it is also a problem figuringhow to adapt the DM-RS density to UEs in MU-MIMO scheduling.

Various embodiments of the techniques and apparatus described hereinaddress one or several of these problems by providing a solution inwhich an adaptable reference signal density is indicated and transmittedto the UE in a structured way, by merging antenna reference signal orantenna ports of lower density. In this way, the density can be adapteddepending on the current needs. For example, in multi-user-MIMO(MU-MIMO) scheduling, some scheduled UEs may need a higher referencesignal density than others. The presently disclosed techniquesfacilitate efficient scheduling in such scenarios. In some of theexample scenarios below, DM-RS is used for a reference signal, but itshould be understood that the techniques are applicable to otherreference signals.

According to the various embodiments described herein, each antenna portthat might be used at any given time is associated with a certainreference signal structure/pattern that is known to both thetransmitting and receiving devices. Thus, within the context of thepresently disclosed techniques, a given antenna port corresponds to aparticular DM-RS pattern, and the terms “DM-RS port” and “DM-RS antennaport” may be understood as referring to a combination of antenna portand DM-RS pattern. Because an antenna port, as that term is used herein,need not necessarily correspond to a single physical antenna port, itwill be appreciated that the labeling of antenna ports is, at least fromthe perspective of the receiving device, arbitrary. A receiving device(e.g., a UE) receiving either a single-layer or multiple-layertransmission needs to be informed via a signaling message (e.g., an“assignment” or “grant”) about the specific port or ports that are used,as well as the rank of the transmission, in order to performdemodulation of the signal transmitted to it. The same thing alsoapplies in uplink transmissions where a UE is receiving a grantindicating the DM-RS and corresponding rank to be used for the uplinktransmission.

In the various embodiments, a wireless device is semi-staticallyconfigured or dynamically indicated from a message (e.g., schedulingdownlink control message), or a combination of both, with a DM-RSpattern selected to support the envelope of the requirements of acurrent population of users. The densification of DM-RS to supportchallenging signal conditions is obtained by merging one or multipleDM-RS (or antenna ports) of lower density into new DM-RS (or antennaports). Effectively, the total number of DM-RS antenna ports (i.e., themaximum possible rank) is reduced, in an adaptable manner.

The signaling of the possibly merged antenna ports and correspondingrank restrictions is conveyed in a physical layer control signalingmessage via a DCI format that is designed to preserve schedulingflexibility in MIMO and in particular in MU-MIMO operations, whileminimizing the required signaling. The densification of the DM-RSpattern that results from the merging of antenna ports is motivated bythe fact that multiplexed UEs can have different SINR conditions. Theadaptive use of antenna ports with different DM-RS patterns provides foran improved ability to match the most appropriate multi-ranktransmissions to UEs with varying signaling conditions.

For the purposes of illustration, the DM-RS structure illustrated inFIG. 3 is used as the baseline configuration for supporting up to 8layers, with various adaptively obtained densified patterns derived fromthis baseline configuration. It should be understood, however, thatother DM-RS structures could be used as the baseline configuration.

FIG. 4 is a block diagram illustrating an example in a first wirelessnode, such as a network node 30 (e.g., base station), which may beconfigured to carry out embodiments of the techniques for adaptabledensification of a reference signal, such as a DM-RS. The network node30 provides an air interface to a wireless device, e.g., an LTE or 5Gair interface or WLAN air interface for downlink transmission and uplinkreception, which is implemented via antennas 34 and a transceivercircuit 36. The transceiver circuit 36 includes transmitter circuits,receiver circuits, and associated control circuits that are collectivelyconfigured to transmit and receive signals according to a radio accesstechnology, for the purposes of providing cellular communication, orWLAN services if necessary. According to various embodiments, cellularcommunication services may be operated according to any one or more ofthe relevant 3GPP cellular standards, including those for 5G. Thenetwork node 30 also includes communication interface circuits 38 forcommunicating with nodes in the core network, other peer radio nodes,and/or other types of nodes in the network.

The network node 30 also includes one or more processing circuits 32that are operatively associated with and configured to control thecommunication interface circuit(s) 38 and/or the transceiver circuit 36.The processing circuit 32 comprises one or more digital processors 42,e.g., one or more microprocessors, microcontrollers, Digital SignalProcessors (DSPs), Field Programmable Gate Arrays (FPGAs), ComplexProgrammable Logic Devices (CPLDs), Application Specific IntegratedCircuits (ASICs), or any combination thereof. More generally, theprocessing circuit 32 may comprise fixed circuitry, or programmablecircuitry that is specially configured via the execution of programinstructions implementing the functionality taught herein, or maycomprise some combination of fixed and programmable circuitry. Theprocessor(s) 42 may be multi-core.

The processing circuit 32 also includes a memory 44. The memory 44, insome embodiments, stores one or more computer programs 46 and,optionally, configuration data 48. The memory 44 provides non-transitorystorage for the computer program 46 and it may comprise one or moretypes of computer-readable media, such as disk storage, solid-statememory storage, or any combination thereof. By way of non-limitingexample, the memory 44 may comprise any one or more of SRAM, DRAM,EEPROM, and FLASH memory, which may be in the processing circuit 32and/or separate from the processing circuit 32. In general, the memory44 comprises one or more types of computer-readable storage mediaproviding non-transitory storage of the computer program 46 and anyconfiguration data 48 used by the network node 30. Here,“non-transitory” means permanent, semi-permanent, or at leasttemporarily persistent storage and encompasses both long-term storage innon-volatile memory and storage in working memory, e.g., for programexecution.

In some embodiments, the processing circuit 32 of the network node 30 isconfigured to select a set of reference signal (e.g., DM-RS) antennaports for use in transmitting data to or receiving data from one or morewireless devices in a given transmit time interval, from a plurality ofsets of reference signal antenna ports that are available for use, wherethe plurality of sets of reference signal antenna ports includereference signal antenna ports having different reference signaldensities in the frequency dimension, the time dimension, or both. Theprocessing circuit 32 of the network node 30 is further configured tosend, to a wireless device, a message (e.g., scheduling message)indicating a reference signal assignment to the wireless device, usingtransceiver circuit 36, where the message includes an indication of theselected set of reference signal antenna ports.

Note that in this and in the other embodiments described herein, themessage discussed here might be one of several messages that togetherinform the wireless device of the reference signal assignment. Forexample, rather than having one large set of 16 port-layer options beingidentified comprehensively in a single downlink control information(DCI), some embodiments may use RRC signaling to transmit part of theinformation. For example, the RRC may identify a subset, say four, ofthe 16 port-layer options, with a subsequent DCI message indicatingexactly which of the port-layer options within that group of fourapplies. This would reduce the DCI signaling bits; the RRC signaling maybe carried out less frequently.

In some instances, the processing circuit 32 is further configured totransmit data to the wireless device (and, optionally, to one or moreother wireless devices, in the case of multi-user MIMO), using thetransceiver circuit 36, where the transmitted data accompanied byreference signal symbols on the indicated/selected set of referencesignal antenna ports, or receive data from the second wireless node,using the selected set of reference signal antenna ports, where usingthe selected set of reference signal antenna ports comprises performingchannel estimation using reference signal symbols on the selected set ofreference signal antenna ports.

In some embodiments, the processing circuit 32 is configured to performa method for scheduling a data transmission to or from a wirelessdevice, such as method 500 shown in FIG. 5. The method 500 includesselecting a set of reference signal (e.g., DM-RS) antenna ports for usein transmitting data to or receiving data from one or more wirelessdevices in a given transmit time interval, from a plurality of sets ofreference signal antenna ports that are available for use (block 502).The plurality of sets of reference signal antenna ports includereference signal antenna ports having different reference signaldensities in the frequency dimension, the time dimension, or both. Themethod 500 further includes sending a message (e.g., scheduling message)to a wireless device indicating a reference signal assignment to thewireless device, where the message includes an indication of theselected set of reference signal antenna ports (block 504).

The message may indicate a number of layers of a multi-layertransmission on which the selected set of reference signal antenna portsare for use in the transmitting or receiving of data. The message mayalso indicate one or more values of a plurality of values, where theindicated one or more values correspond to the selected set of one ormore reference signal antenna ports, and where the plurality of valuescorresponds to reference signal antenna ports having different referencesignal densities and/or a different number of layers on which theselected set of reference signal antenna ports are for use. Theplurality of values may be a single set of values corresponding to allof the plurality of sets of reference signal antenna ports that areavailable for use.

In some cases, the message indicates to the second wireless nodedifferent reference signal densities for different layers in amulti-layer transmission.

In another set of embodiments, the reference signal antenna port of theselected set of reference signal antenna ports may have acode-time-frequency or time-frequency pattern which is orthogonal to afurther reference signal antenna port of the selected set of referencesignal antenna ports having a different reference signal density. Thereference signal antenna ports may comprise a time-frequency patterncomprising a frequency comb, and/or the reference signal antenna portsmay have different densities in the frequency dimension comprisedifferent densities of frequency comb.

The reference signal antenna port of the selected set of referencesignal antenna ports may have a time-frequency pattern which is a mergerof a plurality of time-frequency patterns of a plurality of referencesignal antenna ports of the plurality of sets of reference signalantenna ports that are available for use. The selecting of the set ofreference signal antenna ports may comprise not selecting a referencesignal antenna port having a time-frequency pattern which is merged toprovide a reference signal antenna port of the selected set of referencesignal antenna ports. The reference signal antenna port may have atime-frequency pattern with a higher density is a merger oftime-frequency patterns of first and second reference signal antennaports having lower density patterns. The reference signal antenna portmay have a time-frequency pattern with a still higher density is amerger of time-frequency patterns of third and fourth reference signalantenna ports having the higher density pattern.

The selected set of reference signal antenna ports may comprisereference signal antenna ports having time-frequency patterns ofdifferent density. The reference signal antenna port may correspond to amapping on a code-time-frequency grid.

In some cases, the transmitting or receiving of data is on one or morelayers of a multi-layer Multiple-Input Multiple-Output (MIMO)transmission.

In some cases, as suggested above, sending the message may comprisesending a scheduling message, and/or wherein the reference signalantenna port is a DM-RS antenna port. While some examples may include aDM-RS antenna port and a scheduling message, the techniques describedherein are applicable more generally to other reference signals andmessages.

Additional examples of the method 500 include, in some cases,transmitting data to the wireless device (and, optionally, to one ormore other wireless devices, in the case of multi-user MIMO),accompanied by reference signal symbols (in these examples, DM-RSsymbols) on the indicated/selected set of DM-RS antenna ports. In othercases, the signaling indicates an uplink assignment, and the method 500further comprises receiving data from the wireless device, using theindicated/selected DM-RS antenna ports, where using theindicated/selected DM-RS antenna ports comprises performing channelestimation using DM-RS symbols on the indicated/selected set of DM-RSantenna ports.

In some embodiments, the signaling may comprise two-part signaling. Insuch embodiments, the scheduling message includes a first set of bitsindicating the selected set of DM-RS antenna ports and a second set ofbits indicating which DM-RS antenna ports are assigned to the secondwireless node and indicating a transmission rank associated with theassigned DM-RS antenna ports. The number of bits in the second set ofbits and/or the encoding of the second bits depends on the contents ofthe first set of bits.

FIG. 6 illustrates an example wireless device 50 (e.g., UE) that isconfigured to perform the techniques described herein for the secondwireless node. The wireless device 50 may also be considered torepresent any wireless devices that may operate in a network and thatare capable of communicating with a network node or another wirelessdevice over radio signals. The wireless device 50 may also be referredto, in various contexts, as a radio communication device, a targetdevice, a device-to-device (D2D) UE, a machine-type UE or UE capable ofmachine to machine (M2M) communication, a sensor-equipped UE, a PDA(personal digital assistant), a wireless tablet, a mobile terminal, asmart phone, laptop-embedded equipment (LEE), laptop-mounted equipment(LME), a wireless USB dongle, a Customer Premises Equipment (CPE), etc.

The wireless device 50 communicates with one or more radio nodes or basestations, such as one or more network nodes 30, via antennas 54 and atransceiver circuit 56. The transceiver circuit 56 may includetransmitter circuits, receiver circuits, and associated control circuitsthat are collectively configured to transmit and receive signalsaccording to a radio access technology, for the purposes of providingcellular communication services.

The wireless device 50 also includes one or more processing circuits 52that are operatively associated with and control the radio transceivercircuit 56. The processing circuit 52 comprises one or more digitalprocessing circuits, e.g., one or more microprocessors,microcontrollers, DSPs, FPGAs, CPLDs, ASICs, or any mix thereof. Moregenerally, the processing circuit 52 may comprise fixed circuitry, orprogrammable circuitry that is specially adapted via the execution ofprogram instructions implementing the functionality taught herein, ormay comprise some mix of fixed and programmed circuitry. The processingcircuit 52 may be multi-core.

The processing circuit 52 also includes a memory 64. The memory 64, insome embodiments, stores one or more computer programs 66 and,optionally, configuration data 68. The memory 64 provides non-transitorystorage for the computer program 66 and it may comprise one or moretypes of computer-readable media, such as disk storage, solid-statememory storage, or any mix thereof. By way of non-limiting example, thememory 64 comprises any one or more of SRAM, DRAM, EEPROM, and FLASHmemory, which may be in the processing circuit 52 and/or separate fromprocessing circuit 52. In general, the memory 64 comprises one or moretypes of computer-readable storage media providing non-transitorystorage of the computer program 66 and any configuration data 68 used bythe wireless device 50.

Accordingly, in some embodiments, the processing circuit 52 of thewireless device 50 is configured to, as a second wireless node, use thetransceiver circuit 56 to receive a message (e.g., scheduling message)indicating a reference signal (e.g., DM-RS) assignment to the UE, wherethe message includes an indication of a set of reference signal antennaports selected from a plurality of available sets of reference signalantenna ports known to the UE, where the plurality of sets of referencesignal antenna ports include reference signal antenna ports havingdifferent reference signal densities in the frequency dimension and/ortime dimension. The processing circuit 52 is further configured toidentify the indicated set of reference signal antenna ports from thereceived indication.

In some cases, the processing circuit 52 is further configured toreceive data via the transceiver circuit 56, using theindicated/selected reference signal antenna ports, where using theindicated/selected reference signal antenna ports comprises performingchannel estimation using reference signal symbols on theindicated/selected set of reference signal antenna ports. In othercases, the signaling indicates an uplink assignment, and the processingcircuit 52 is further configured to use the transceiver circuit 56 totransmit data, accompanied by reference signal symbols on theindicated/selected set of reference signal antenna ports.

In some embodiments, the wireless device 50 is configured to perform amethod 700, as shown in FIG. 7. The method 700 includes receiving ascheduling message indicating a reference signal assignment to thewireless device, where the scheduling message includes an indication ofa set of reference signal antenna ports selected from a plurality ofavailable sets of reference signal antenna ports known to the wirelessdevice, and where the plurality of sets of reference signal antennaports include reference signal antenna ports having different referencesignal densities in the frequency dimension and/or time dimension (block702). The method 700 further comprises identifying the indicated set ofreference signal antenna ports from the received indication (block 704).

The message may indicate a number of layers of a multi-layertransmission on which the selected set of reference signal antenna portsare for use in the transmitting or receiving of data. The message mayalso indicate one or more values of a plurality of values, where theindicated one or more values correspond to the selected set of one ormore reference signal antenna ports, and where the plurality of valuescorresponds to reference signal antenna ports having different referencesignal densities and/or a different number of layers on which theselected set of reference signal antenna ports are for use. Theplurality of values may be a single set of values corresponding to allof the plurality of sets of reference signal antenna ports that areavailable for use.

In some cases, the message indicates to the second wireless nodedifferent reference signal densities for different layers in amulti-layer transmission.

In another set of embodiments, the reference signal antenna port of theselected set of reference signal antenna ports may have acode-time-frequency or time-frequency pattern which is orthogonal to afurther reference signal antenna port of the selected set of referencesignal antenna ports having a different reference signal density. Thereference signal antenna ports may comprise a time-frequency patterncomprising a frequency comb, and/or the reference signal antenna portsmay have different densities in the frequency dimension comprisedifferent densities of frequency comb. The reference signal antenna portof the selected set of reference signal antenna ports may have atime-frequency pattern which is a merger of a plurality oftime-frequency patterns of a plurality of reference signal antenna portsof the plurality of sets of reference signal antenna ports that areavailable for use. The selecting of the set of reference signal antennaports may comprise not selecting a reference signal antenna port havinga time-frequency pattern which is merged to provide a reference signalantenna port of the selected set of reference signal antenna ports. Thereference signal antenna port may have a time-frequency pattern with ahigher density is a merger of time-frequency patterns of first andsecond reference signal antenna ports having lower density patterns. Thereference signal antenna port may have a time-frequency pattern with astill higher density is a merger of time-frequency patterns of third andfourth reference signal antenna ports having the higher density pattern.

The selected set of reference signal antenna ports may comprisereference signal antenna ports having time-frequency patterns ofdifferent density. The reference signal antenna port may correspond to amapping on a code-time-frequency grid.

In some cases, the method 700 further comprises receiving data, usingthe indicated/selected reference signal antenna ports, where using theindicated/selected reference signal antenna ports comprises performingchannel estimation using reference signal symbols on theindicated/selected set of reference signal antenna ports. In othercases, the signaling indicates an uplink assignment, and the method 700further comprises transmitting data, accompanied by reference signalsymbols on the indicated/selected set of reference signal antenna ports.In some cases, the transmitting or receiving of data is on one or morelayers of a multi-layer MIMO transmission.

In some cases, as suggested above, sending the message may comprisesending a scheduling message, and/or wherein the reference signalantenna port is a DM-RS antenna port. As previously stated, while someexamples include a DM-RS antenna port and a scheduling message, thetechniques described herein are applicable more generally to otherreference signals and messages. In some embodiments, the signaling maycomprise two-part signaling. For instance, a first bit or a set of bitsmay be used to indicate to a wireless device how the reference signal(e.g., DM-RS) are mapped, or densified, on a time-frequency grid relatedto an OFDM-based transmission, relative to a particular baselineconfiguration that is already known to the UE. A second set of bits maybe used to indicate the actual rank and/or choice of transmission ports.This densifying and signaling procedure is exemplified below in FIGS. 9through 11 with a corresponding signaling given by Table 1 of FIG. 8A.DM-RS is used as a representative reference signal in these examples.

From the baseline pattern shown in the upper part of FIG. 9, it can beseen that only three resource elements per layer (antenna port) perphysical resource block (PRB) are used in the frequency domain forDM-RS. In some scenarios, this will be an insufficient density foraccurate demodulation.

One method to obtain a denser pattern for some layers of the DM-RS combstructure in FIG. 9 is to merge ports 0 & 4 and ports 1 & 5 into two newports, here referred to as ports 8 and 9. This merger is shown in thebottom part of FIG. 9. As seen in the left-hand side of FIG. 9, at thebottom, there are twice as many resource elements used for ports 8 and 9as are used for either ports 0 & 4 or ports 1 & 5 in the baselineconfiguration at the top. Moving to the right in FIG. 9, it can be seenthat ports 0, 1, 4, and 5 are not used after this merger. The DM-RS forports 2, 3, 6, and 7 remain the same, in this particular example. Notethat the port numbering/labeling and the choice of which pairs of portsto merge is arbitrary, and may vary from one design to another. FIG. 9simply illustrates one example, where two pairs of antenna ports andtheir corresponding DM-RS are merged, to form a new pair of antennaports having DM-RS that are twice as dense.

A consequence of this merge is that it will not be possible to assignhigher than rank 6 to any user. Note, however, that this implies thatfewer bits are needed for this signaling. Another consequence of thismerging of two pairs of ports is that the ports in the resulting OFDMsymbol/slots have unequal DM-RS densities. The ports 8 and 9 have higherport density than the remaining ports at the cost of supporting a lowernumber of maximum ports. This actually provides several advantages.First, the ports having higher DM-RS densities can be used totransmit/demodulate transmitted information that is more sensitive toerrors, such as control information or HARQ-ACK information, or datathat has higher reliability such as the ultra-reliable and low latencyuse cases (URLLC) that are expected to be important in next-generationwireless systems.

The example densification shown in FIG. 9 can be extended, to providedensification of other pairs of ports and/or to provide even furtherdensification. FIG. 10 and FIG. 11 show the procedure of furtherdensifying the DM-RS pattern and restricting the available ports whengoing from highest rank of 6 to rank 4 and when going from highest rankof 4 to rank 2, respectively.

More particularly, FIG. 10 shows how the densification shown in FIG. 9is extended to include the merging of ports 2 & 6 with ports 3 & 7, toyield new ports 10 & 11, with correspondingly densified DM-RS. Again, itcan be seen that ports 10 & 11 have DM-RS with twice the density of theDM-RS for either ports 2 & 6 or ports 3 & 7. The resultingconfiguration, however, supports only up to 4-layer MIMO transmission,as there are only four ports (ports 8, 9, 10, 11) and correspondingDM-RS available.

FIG. 11 shows how the same approach can be extended to provide evenfurther densification. The top portion of FIG. 11 shows the result ofgoing from a 6-port configuration to a 4-port configuration, as wasshown in FIG. 10. The bottom portion of FIG. 11 shows the merger ofports 8 & 9 with ports 10 & 11, to yield new ports 12 & 13. These newports have four times the density of any of the starting ports 0-7, andtwice the density of ports 8-11. The result, however, is a configurationthat supports the transmission of only one or two layers.

It will be appreciated that the various densification patterns shown inFIGS. 9-12 can be used to provide a UE with different densities of DM-RSfor different layers in a multi-layer transmission, for example. Thesevarious patterns can also or instead be used to provide different UEswith different DM-RS densities, for MU-MIMO transmissions.

The above described techniques of densifying a comb-based DM-RS patternis one example of how to merge different ports that is particularlysuitable for when a low cubic metric is needed and an advanced channelestimator is feasible. These techniques may be used in the uplink in aradio network, for example, where a low power UE is transmitting and anadvanced base-station is doing the channel estimation. In some otherembodiments, the merging of pairs of antenna ports and theircorresponding DM-RS is instead performed using pairs of antenna portsthat use adjacent resource elements for their DM-RS, as depicted in theexample configuration shown in FIG. 12. As seen in FIG. 12, antennaports 0 & 4 and their DM-RS's are merged with antenna ports 2 & 6 andtheir DM-RS's, to obtain new ports 8 & 9. With this pairing/merging, theresulting DM-RS for ports 8 & 9 occupy pairs of immediately adjacent (inthe frequency-domain) resource elements.

The approach shown in FIG. 12 might be used, for example, in thedownlink, when a base station is transmitting to a set of multiple UEsand a simpler channel estimation procedure is desired for the UE, e.g.,due to power and computational restrictions. FIG. 12 shows thedensification and port restriction of going from highest rank 8 tohighest rank 6. The procedure of further densifying the DM-RS patternand restricting the available ports of FIG. 12 follow the same principleas discussed above, and as illustrated with FIGS. 10 and 11.

Another use case for the selective use of adaptively denser DM-RS portscould be the use of shared reference signals, where these referencesignals are potentially wideband, and shared among multiple users servedby a transmitting node. As suggested above, still another use case forunequal DM-RS density is to adapt the DM-RS density in MU-MIMOscheduling, where one user may have worse SINR conditions and needs moreenergy for DM-RS compared to users that have better SINR.

As briefly noted above, the increased densification of DM-RS accordingto the structured techniques described herein corresponds to a decreasein the number of antenna ports that are supported, and thus to adecrease in the maximum possible rank for a transmission. Thiscorrelation, between increased densification and reduced maximum rank,can be exploited to minimize the number of bits needed to signal theDM-RS pattern that is in use at any given time and the correspondingmaximum rank.

Table 1 in FIG. 8A illustrates an example approach to signaling in whichtwo bits are used to indicate both the set of available ports as well asthe associated rank restriction, for a certain DM-RS pattern. It can beobserved that this table does not specify the particular mappingfunction for rank and DM-RS; rather the table simply illustrates thatthe number of bits for signaling this information scales with the numberof possible available DM-RS, i.e., with the maximum supported rank.

In some embodiments, an indication of the number of layers and whichspecific ports to use when receiving a data transmission is indicated tothe UE in the scheduling DCI message. This explicit signaling allows theuse of MU-MIMO scheduling, using orthogonal antenna ports betweendifferent UEs, where the used ports may have different DM-RS densities,as exemplified in FIG. 9 to FIG. 11, where ports 0-7 have the lowestDM-RS density, ports 8-11 have medium DM-RS density, and ports 12-13have the highest DM-RS density.

Note that terminology from LTE is being used here, to refer to thedownlink message that carries scheduling information, i.e., informationindicating granted downlink time-frequency resources and indicating themodulation and coding scheme to be applied to data transmitted in thoseresources. Systems employing the presently disclosed techniques may usescheduling messages described using different terminology, and havingdifferent formats than those used in LTE.

In some embodiments, the following MU-MIMO scheduling examples arepossible, but not necessarily exclusive. Note that the “Values” providedbelow refer to entries in the table of FIG. 8B, which may in severalembodiments be signaled to the UE in a DCI message. In cases ofsimultaneously scheduling UE1 and UE2 with the same port density,examples may include: two rank-1 transmissions (e.g., ports 0 and 1:Values 0 and 1); two rank-2 transmissions (e.g., port 0-1 and 2-3:Values 4 and 5); or two rank-2 transmission (e.g., port 8-9 and 10-11:Values 11 and 13). In cases of simultaneously scheduling UE1 and UE2with different port densities, examples may include two rank-1transmission (e.g., ports 2 and 8: Value 2 and 1); or a rank-2 and arank-4 transmission (e.g., ports 8-9 and 2-3-6-7: Values 11 and 9).

It can also be seen from Table 2 of FIG. 8B that for a single UE, it ispossible to schedule rank-1 or rank-2 transmissions with any of threedifferent DM-RS densities, corresponding to, for example, Values 0, 10,and 14 for rank 1, and to Values 4, 11, and 15 for rank 2, using thedynamic indications provided by signaling the “Value” parameter to theUE.

With the above description and detailed examples in mind, it will beappreciated that FIG. 13 is a process flow diagram illustrating anexample method 1300 for decoding and applying signaling bits thatindicate both a mapping of reference signals (DM-RS signals in thisexample process) for various antenna ports to resource elements of areceived signal and a maximum number of DM-RS ports (i.e., a maximumpossible rank for multi-layer transmission). As shown at block 1302, theillustrated method 1300 begins with decoding a first set of bits, e.g.,in a DCI message, where the first set of bits indicate a mapping ofports to resource elements. More specifically, the first set of bitsindicate a particular set of available ports, from a plurality ofpossible sets of available ports, as well as indicating the density ofthe DM-RS for each port in the set. As was illustrated in Table 1 ofFIG. 8A, this can be done in a system that supports up to 8-layertransmission using only two bits, assuming that there is apre-determined association between the possible bit sequences and thesets of available ports. As shown in FIG. 8A, for example, a two-bitsequence of 00 would indicate that the available port set consists ofports 0-7, while a two-bit sequence of 11 would indicate that theavailable port set consists of only ports 12 and 13. The receivingdevice (e.g., a UE) would know from previously configured informationthat ports 0-7 have particular (low-density) DM-RS patterns, while ports12 and 13 have different, and higher-density DM-RS patterns.

As shown at block 1304, the maximum number of DM-RS ports (and hence themaximum rank for transmission) can be determined from the first set ofbits. Using the mapping in FIG. 8A, for example, a receiving devicewould know that bit sequence 00 indicates a maximum supported rank of 8,while bit sequence 11 indicates a maximum supported rank of 2.

At this point, the receiving device knows the transmitted DM-RSpatterns, and their mapping to antenna ports, but does not knowspecifically which ports are scheduled for the receiving device. Thislatter information can be signaled with a second set of bits. Becausethe first set of bits indicates the maximum supported rank, the mostefficient coding of the second of bits will depend on the content of thefirst set of bits, or, equivalently, on the maximum supported rankindicated by the first set of bits. For example, if the first set ofbits indicates a maximum supported rank of two (e.g., with a bitsequence of 11, as shown in FIG. 8A), then the second set of bits needonly include two bits to signal that either port 12 or 13 (for a rank-1transmission), or both (for a rank-2 transmission), are scheduled forthe receiving device. On the other hand, if the first two bits indicatethat up to 8-layer transmissions are available, e.g., using some or allof ports 0-7 as shown in the top portion of FIG. 9, then four or fivebits may be needed to indicate the rank of a scheduled transmission andthe corresponding ports, depending on how many different combinations ofports are permitted for each allowed transmission rank. Similarly, ifthe first two bits indicate that only up to 4-layer transmissions areavailable, e.g., using ports 8-11 as shown in the bottom portion of FIG.10, then perhaps three or four bits may be needed to indicate the rankof a scheduled transmission and the corresponding ports, again dependingon how many different combinations of ports are permitted for eachallowed transmission rank (one to four, in this scenario).

Accordingly, it is seen that both the number of bits and the specificencoding of the bits in the second set of bits may depend on the valueconveyed in the first set of bits, in some signaling schemes accordingto the presently disclosed techniques. Thus, block 1306 of FIG. 13indicates that the illustrated method includes deducing, from themaximum number of ports indicated by the first set of bits, both thenumber and the specific encoding scheme for bits in a second set ofreceived bits. As shown at block 1308, the contents of this second setof bits (which may vary in number, depending on the value carried by thefirst bits) are then mapped to a specific set of DM-RS ports. Asindicated above, this mapping is dependent on the value carried by thefirst set of bits, in some embodiments. It will be appreciated that thisapproach provides for a great deal of flexibility in indicating specificDM-RS patterns, of adaptable densities, along with an indication oftransmission rank and specifically used ports, while also allowing thenumber of signaled bits to be kept as low as possible.

Note also that while methods 500 is described as being carried out in anetwork node such as a base station, in communication with a wirelessdevice, and while methods 700 and 1300 are described as being carriedout in a wireless device, the methods 500, 700 and 1300 are moregenerally applicable to first and second wireless nodes. In some cases,the network node 30 may be configured to perform methods 700 and 1300,and the wireless device 50 may be configured to perform method 500.

As discussed in detail above, the techniques described herein, e.g., asillustrated in the process flow diagrams of FIGS. 5, 7 and 13, may beimplemented, in whole or in part, using computer program instructionsexecuted by one or more processors. It will be appreciated that afunctional implementation of these techniques may be represented interms of functional modules, where each functional module corresponds toa functional unit of software executing in an appropriate processor orto a functional digital hardware circuit, or some combination of both.

For example, FIG. 14 is a block diagram illustrating a functionalimplementation as carried out in a first wireless node, or a networknode 30 such as a base station. The implementation includes a selectingmodule 1402 for selecting a set of reference signal (e.g., DM-RS)antenna ports for use in transmitting data to or receiving data from oneor more other wireless nodes in a given transmit time interval, from aplurality of sets of reference signal antenna ports that are availablefor use, where the plurality of sets of reference signal antenna portsinclude reference signal antenna ports having different reference signaldensities in the frequency dimension, the time dimension, or both. Theimplementation also includes a sending module 1404 for sending a message(e.g., scheduling message) to a second wireless node indicating areference signal assignment to the second wireless node, wherein themessage includes an indication of the selected set of reference signalantenna ports.

FIG. 15 is a block diagram illustrating a functional implementation ascarried out in a second wireless node, or a wireless device 50 such as aUE. The implementation includes a receiving module 1502 for receiving,from a first wireless node, a message indicating a reference signalassignment to the second wireless node, wherein the message includes anindication of a set of reference signal antenna ports selected from aplurality of available sets of reference signal antenna ports known tothe second wireless node, and wherein the plurality of sets of referencesignal antenna ports include reference signal antenna ports havingdifferent reference signal densities in the frequency dimension and/ortime dimension. The implementation also includes an identifying module1504 for identifying the indicated set of reference signal antenna portsfrom the received indication.

Notably, modifications and other embodiments of the disclosedinvention(s) will come to mind to one skilled in the art having thebenefit of the teachings presented in the foregoing descriptions and theassociated drawings. Therefore, it is to be understood that theinvention(s) is/are not to be limited to the specific embodimentsdisclosed and that modifications and other embodiments are intended tobe included within the scope of this disclosure. Although specific termsmay be employed herein, they are used in a generic and descriptive senseonly and not for purposes of limitation.

1. A method, in a first wireless node, the method comprising: selectinga set of reference signal antenna ports for use in transmitting data toor receiving data from one or more other wireless nodes in a giventransmit time interval, from a plurality of sets of reference signalantenna ports that are available for use, wherein the plurality of setsof reference signal antenna ports include reference signal antenna portshaving different reference signal densities in the frequency dimension,the time dimension, or both; and sending a message to a second wirelessnode indicating a reference signal assignment to the second wirelessnode, wherein the message includes an indication of the selected set ofreference signal antenna ports.
 2. The method of claim 1, wherein themessage indicates a number of layers of a multi-layer transmission onwhich the selected set of reference signal antenna ports are for use inthe transmitting or receiving of data.
 3. The method of claim 1, whereinthe message indicates one or more values of a plurality of values,wherein the indicated one or more values correspond to the selected setof one or more reference signal antenna ports, and wherein the pluralityof values corresponds to reference signal antenna ports having differentreference signal densities and/or a different number of layers on whichthe selected set of reference signal antenna ports are for use.
 4. Themethod of claim 3, wherein the plurality of values is a single set ofvalues corresponding to all of the plurality of sets of reference signalantenna ports that are available for use.
 5. The method of claim 1,wherein the message indicates to the second wireless node differentreference signal densities for different layers in a multi-layertransmission.
 6. The method of claim 1, wherein sending the messagecomprises sending a scheduling message, and/or wherein the referencesignal antenna port is a demodulation reference signal, DM-RS, antennaport.
 7. The method of claim 1, wherein the transmitting or receiving ofdata is on one or more layers of a multi-layer Multiple-InputMultiple-Output, MIMO, transmission.
 8. The method of claim 1, whereinthe reference signal antenna port corresponds to a mapping on acode-time-frequency grid.
 9. A first wireless node, wherein the firstwireless node is adapted to: select a set of reference signal antennaports for use in transmitting data to or receiving data from one or moreother wireless nodes in a given transmit time interval, from a pluralityof sets of reference signal antenna ports that are available for use,wherein the plurality of sets of reference signal antenna ports includereference signal antenna ports having different reference signaldensities in the frequency dimension, the time dimension, or both; andsend a message to a second wireless node indicating a reference signalassignment to the second wireless node, wherein the message includes anindication of the selected set of reference signal antenna ports.10.-16. (canceled)
 17. The first wireless node of claim 9, wherein thefirst wireless node is a base station and the second wireless node is auser equipment, UE.
 18. A method, in a second wireless node, the methodcomprising: receiving, from a first wireless node, a message indicatinga reference signal assignment to the second wireless node, wherein themessage includes an indication of a set of reference signal antennaports selected from a plurality of available sets of reference signalantenna ports known to the second wireless node, and wherein theplurality of sets of reference signal antenna ports include referencesignal antenna ports having different reference signal densities in thefrequency dimension, the time dimension, or both; and identifying theindicated set of reference signal antenna ports from the receivedindication.
 19. The method of claim 18, wherein the message indicates anumber of layers of a multi-layer transmission on which the selected setof reference signal antenna ports are for use in the transmitting orreceiving of data.
 20. The method of claim 18, wherein the messageindicates one or more values of a plurality of values, wherein theindicated one or more values correspond to the selected set of one ormore reference signal antenna ports, and wherein the plurality of valuescorresponds to reference signal antenna ports having different referencesignal densities and/or a different number of layers on which theselected set of reference signal antenna ports are for use.
 21. Themethod of claim 20, wherein the plurality of values is a single set ofvalues corresponding to all of the plurality of sets of reference signalantenna ports that are available for use.
 22. The method of claim 18,wherein the message indicates to the second wireless node differentreference signal densities for different layers in a multi-layertransmission.
 23. The method of claim 18, wherein the message comprisesa scheduling message, and/or wherein the reference signal antenna portis a demodulation reference signal, DM-RS, antenna port.
 24. The methodof claim 18, wherein the reference signal antenna port corresponds to amapping on a code-time-frequency grid.
 25. The method of claim 18,further comprising receiving data, using the indicated reference signalantenna ports, wherein using the indicated reference signal antennaports comprises performing channel estimation using reference signalsymbols on the indicated set of reference signal antenna ports.
 26. Themethod of claim 18, further comprising transmitting data to the firstwireless node, accompanied by reference signal symbols on the indicatedset of reference signal antenna ports.
 27. The method of claim 25,wherein the transmitting or receiving of data is on one or more layersof a multi-layer Multiple-Input Multiple-Output, MIMO, transmission. 28.A second wireless node, wherein the second wireless node is adapted to:receive, from a first wireless node, a message indicating a referencesignal assignment to the second wireless node, where the messageincludes an indication of a set of reference signal antenna portsselected from a plurality of available sets of reference signal antennaports known to the second wireless node, wherein the plurality of setsof reference signal antenna ports include reference signal antenna portshaving different reference signal densities in the frequency dimension,the time dimension, or both; and identify the indicated set of referencesignal antenna ports from the received indication. 29.-37. (canceled)38. The second wireless node of claim 28, wherein the second wirelessnode is a user equipment, UE, and the first wireless is a base station.39. A first wireless node configured for operation in a wirelesscommunication network, wherein the first wireless node comprises atransceiver circuit and a processing circuit operatively coupled to thetransceiver circuit, wherein the processing circuit is configured toselect a set of reference signal antenna ports for use in transmittingdata to or receiving data from one or more other wireless nodes in agiven transmit time interval, from a plurality of sets of referencesignal antenna ports that are available for use, where the plurality ofsets of reference signal antenna ports include reference signal antennaports having different reference signal densities in the frequencydimension and/or time dimension, and the processing circuit isconfigured to send to a second wireless node, via the transceivercircuit, a message indicating a reference signal assignment to thesecond wireless node, where the message includes an indication of theselected set of reference signal antenna ports.
 40. A second wirelessnode configured for operation in a wireless communication network,wherein the second wireless node comprises a transceiver circuit and aprocessing circuit operatively coupled to the transceiver circuit,wherein the processing circuit is configured to receive from a firstwireless node, via the transceiver circuit, a message indicating areference signal assignment to the second wireless node, where themessage includes an indication of a set of reference signal antennaports selected from a plurality of available sets of reference signalantenna ports known to the second wireless node, and the plurality ofsets of reference signal antenna ports include reference signal antennaports having different reference signal densities in the frequencydimension and/or time dimension, and the processing circuit isconfigured to identify the indicated set of reference signal antennaports from the received indication.